Offshore oil and gas operations often utilize a wellhead housing supported on the ocean floor and a blowout preventer stack secured to the wellhead housing's upper end. A blowout preventer stack is an assemblage of blowout preventers and valves used to control well bore pressure. The upper end of the blowout preventer stack has an end connection or riser adapter (often referred to as a lower marine riser package or LMRP) that allows the blowout preventer stack to be connected to a series of pipes, known as riser, riser string, or riser pipe. Each segment of the riser string is connected in end-to-end relationship, allowing the riser string to extend upwardly to the drilling rig or drilling platform positioned over the wellhead housing.
The riser string is supported at the ocean surface by the drilling rig and extends to the subsea equipment through a moon pool in the drilling rig. A rotary table and associated equipment typically support the riser string during installation. Below the rotary table may also be a diverter, a riser gimbal, and other sensitive equipment.
During installation of the riser string, it may be necessary to temporarily move the entire drilling rig, such as for example when a strong storm is approaching Before moving the rig, it is necessary to pull up the entire riser. If the riser were left in place, movement of the rig would cause the riser string to damage the rotary table, diverter, gimbal, and other sensitive equipment. Pulling up each section of riser string takes a long time, adding cost to the overall drilling operations. Additionally, there may not be enough time to pull the entire riser string before the rig needs to be moved.